The paper proposes a novel method called IMO (Invariant features Masks for Out-of-Distribution text classification) to achieve out-of-distribution (OOD) generalization for text classification tasks. The key idea is to learn sparse domain-invariant representations from pre-trained transformer-based language models in a greedy layer-wise manner.
During training, IMO learns sparse mask layers to remove irrelevant features for prediction, where the remaining features are invariant across domains. Additionally, IMO employs a token-level attention mechanism to focus on the tokens that are most useful for prediction.
The authors provide a theoretical analysis to elucidate the relationship between domain-invariant features and causal features, and explain how IMO learns the invariant features.
The comprehensive experiments show that IMO significantly outperforms strong baselines, including prompt-based methods and large language models, on various evaluation metrics and settings for both binary sentiment analysis and multi-class classification tasks. IMO also demonstrates better performance when the size of the training data is limited, indicating its effectiveness in low-resource scenarios.
The authors also conduct ablation studies to justify the effectiveness of the top-down greedy search strategy and the individual components of IMO, such as the mask layers and attention mechanism.
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